You'll have concentration of the existing alpha acids, so it will be roughly 20% more bitter than at 6 gal. I suspect you'll also get increased utilization, but even going from 60 minutes to 120, for example, isn't a huge increase. If there were large late hop additions, that might be different. A 20 minute addition would effectively become an 80 minute, and so on.

Along with color, you'll also get increased production of Maillard products, and I'm sure that will have perceptible flavor effects.

I think the only effect you'll be looking at is the increased extraction due to the lower gravity wort for the initial length of the boil, and then concentration of the extracted bitterness as you boiled further. So let's say you extracted 20 IBU while the hops were in there, 20*6/5 = 24 IBU, or as Sean said "You'll have concentration of the existing alpha acids, so it will be roughly 20% more bitter than at 6 gal." However, if you had originally boiled it in a higher gravity wort you would have extracted 22.1 IBU, so in the end it will only be a little more bitter than you intended and I don't think you'll notice. This is based on some numbers I ran in beersmith with a gravity of 1.049 for 6 gallons (1.060 for 5 gallons) and enough hops at 60 minutes to get 20 IBUs in 6 gallons.

Hey Marc, I hope you are well. Was not really worried but it was the first time I've cooled and then re-boiled. Had to see if I was missing something.

I'm doing pretty well, but I won't be seeing everyone in Michigan this year. Hope to see y'all in San Diego next year. I figure the only difference you might notice is a lack of flavor, as some of the flavor compounds may be boiled off. I have a nasty tendency to run out of gas in the middle of a boil, and I haven't noticed too much of an issue.